Gear differentials generally include compound planetary gear sets interconnecting a pair of drive axles to permit the latter to rotate in opposite directions with respect to a differential housing. The drive axles rotate about a common axis; and so-called "side" gears are fixed for rotation with the inner ends of the two drive axles, such side gears acting as the sun gear members of the compound planetary gear sets. The sun gears are interconnected by so-called "element" or "combination" gears, which form the planet gear members of the sets. The planet gears are mounted for rotation about axes that may be variously offset and inclined with respect to the common axis of the sun gears and drive shafts.
The relative positions of the sun and planet gear axes usually determine the kind of gearing that make up the planetary gear sets: Parallel axes are used for mounting spur or helical gears, e.g., see U.S. Pat. Nos. 2,269,734 (Powell); 2,972,265 (Walter); and 3,095,761 (Hilado); and orthogonal axes are used for mounting either bevel or worm gears, depending upon the presence of any offset between the axes. That is, bevel gears are used when the sun and planet gear axes intersect, while worm gears are used when the gear axes do not intersect (as an example of this latter type, see U.S. Pat. No. 1,373,657 to Finefrock).
The entire planetary gearing arrangement within the differential supports opposite relative rotation between the drive axle ends (i.e., differentiation), which is necessary to permit the axle ends to be driven at different speeds. Torque transmitted to the drive axles through the inclined tooth surfaces of the sun gears generates thrust forces against gear-mounting bearing surfaces within the differential. (Such bearing surfaces may comprise journals formed in the housing, or may be the ends of bores into which the gears are received, or may be special washers positioned between the end faces or shaft ends of the gears and the housing.) The thrust forces, together with other loads conveyed by the gear meshes in the planetary gearing, produce a frictional resistance to relative rotation between the drive axles, this frictional resistance being proportional to the torque applied to the differential housing. The proportional frictional resistance supports different amounts of torque between the two drive axles to prevent their relative rotation until the characteristic "bias" ratio of the planetary gearing arrangement is reached. Once the frictional resistance is overcome and differentiation begins, the torque difference between the axles is proportioned in accordance with the bias ratio. Differentials that divide torque in a substantially constant ratio between relatively rotating drive axles are referred to as "torque-proportioning" differentials.
The ability to support different amounts of torque between the drive axles is of great benefit to improving traction capabilities of vehicles. Ordinarily, when one wheel of a vehicle with a conventional differential loses traction, the amount of torque that can be delivered to the other drive wheel is similarly reduced. However, when one wheel loses traction so that there is differentiation between the two axles, torque-proportioning differentials deliver an increased amount of torque to the drive wheel having better traction, such increased torque being determined by the characteristic bias ratio of the differential.
A wide variety of torque-proportioning differentials were developed more than seventy-five years ago to facilitate automotive travel over the muddy, unpaved roads that were used to supply the trenches in World War I; and these early designs have been improved over and over again since that time. Some well-known designs use planetary gearing assemblies with orthogonal axes (e.g., the above-cited Finefrock Patent), while others use gearing assemblies with parallel axes. Examples of the latter type are the above-cited Powell, Walter, and Hilado Patents as well as U.S. Pat. Nos. 1,195,314 (Williams); 2,000,223 (DuPras); 2,462,000 (Randall); and, more recently, 3,706,239 (Myers), this latter design supporting the gears in housing pockets rather than by shafts received in conventional journal bores.
In general, the gears used in parallel-axis/helical gear assemblies are often simpler to manufacture than are the gears used in torque-proportioning designs of the orthogonal-axis/worm-gear type. However, when the latter are made with the same number of sun and planet elements as the former, they usually develop greater frictional resistance between their respective gear meshes and support bearings; and this, in turn, provides greater torque bias and/or increased control over the bias ratio. A significant portion of the torque bias of such orthogonal-axis designs is related to the frictional resistance developed by the cumulative axial forces (hereinafter referred to as "end thrust") developed by their sun gears and by their planet gears when the differential is subjected to torque. In contrast, such cumulative end thrust has not heretofore been a significant contributor to the torque bias developed by parallel-axis designs.
That is, even though there are several basic prior-art designs for parallel-axis differentials of the torque-proportioning type which make use of the frictional resistance generated by end thrust, we are aware of none that utilizes cumulative end thrust developed by both sun and planet gears for the purpose of creating a substantial portion of the torque bias between the axles. For instance, one of these basic designs is exemplified by the above-cited U.S. Patents to DuPras, Powell, Randall, Walter, and Myers. In this basic prior-art design, helical sun gears of opposite hand are in mesh with one or more pairs of helical planet combination gears which, in turn, are in mesh with each other. While this design produces end thrust on the sun gears, no significant end thrust is developed by the planet gear pairs due to the fact that the end thrust created by the helical teeth in mesh with the sun gear is opposed by a contrary end thrust created by the same hand helical teeth which are used for the interconnection between the planet gear pair.
A second basic parallel-axis design is exemplified by the torque-proportioning differentials disclosed in the above-cited U.S. Patents issued to Williams and Hilado. In this second basic design, the pairs of planet gears mesh with each other by means of helical gearing, while using spur gear teeth for the meshing connection with their respective sun gears. This prior-art arrangement produces end thrust on the planet combination gears, but it does not develop any significant end thrust on the sun gears.
There are other known designs for parallel-axis/torque-proportioning differentials in which, instead of mounting the planet gears in pairs, the gears are mounted in a continuous circular mesh around the full circumference of each respective side gear, e.g., see U.S. Pat. No. 3,738,192 (Belansky). However, like the two basic designs just discussed above, none of these continuous circular mesh designs is directed to controlling the cumulative end thrust independently developed by both sun and planet gears for the purpose of creating a substantial portion of the torque bias.
The invention claimed herein improves such parallel-axis torque proportioning differentials by balancing the load on the planetary gear mounting supports, by increasing the number of frictional surfaces, and by maximizing utilization of the cumulative end thrust developed by both sun and planet gears for the purpose of creating a significant portion of the differential's torque bias, thereby permitting increased control over bias ratio.